1. Field of the Invention
This invention relates generally to methods of producing single-wall carbon nanotubes, and to catalysts for use in such methods.
2. Description of Related Art
Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), “Large-Scale Synthesis Of Carbon Nanotubes,” Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), “Carbon Nanotubes,” Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having multiple walls have been described in the prior art. Ebbesen II; Iijima et al., “Helical Microtubules Of Graphitic Carbon,” Nature, Vol. 354, p. 56 (Nov. 7, 1991).
Another known way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as part of nanotubes growing from one side of the particle. This procedure typically produces imperfect multi-walled nanotubes in high yield. See C. E. Snyder et al., International Patent Application WO 89/07163 (1989), hereby incorporated by reference in its entirety. Its advantage is that it is relatively simple and can be scaled to produce nanotubes by the kilogram.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., “Single-Shell Carbon Nanotubes of 1 nm Diameter,” Nature, Vol. 363, p. 603 (1993); Bethune et al., “Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,” Nature, Vol. 63, p. 605 (1993); Ajayan et al., “Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,” Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., “Single-Walled Carbon Nanotubes Growing Radially From YC2 Particles,” Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., “Single-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,” Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., “Carbon Nanocapsules Encaging Metals and Carbides,” J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., “Extrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,” Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., “Improving Conditions Toward Isolating Single-Shell Carbon Nanotubes,” Chem. Phys. Lett., Vol. 226, p. 364 (1994). While this arc discharge process can produce single-wall nanotubes, the yield of nanotubes is low and the tubes exhibit significant variations in structure and size between individual tubes in the mixture. Individual carbon nanotubes are difficult to separate from the other reaction products and purify.
High quality single-wall carbon nanotubes have also been generated by arc evaporation of a graphite rod doped with Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby incorporated by reference in its entirety. These techniques allow production of only gram quantities of single-wall carbon nanotubes.
An improved method of producing single-wall nanotubes is described in U.S. Ser. No. 08/687,665, entitled “Ropes of Single-Walled Carbon Nanotubes” incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. See A. Thess et al. (1996), Science 273:483. The single-wall nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of 10 to 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. These high quality samples have for the first time enabled experimental confirmation of the structurally dependent properties predicted for carbon nanotubes. See J. W. G. Wildoer, L C. Venema, A. G. Rinzler, R. E. Smalley, C Dekker (1998), Nature, 391:59; T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber (1998), Nature, 391:62.
Although the laser vaporization process produces improved single-wall nanotube preparations, the product is still heterogeneous, and the nanotubes are too tangled for many potential uses of these materials. In addition, the vaporization of carbon is a high-energy process and is inherently costly. Therefore, there remains a need for improved methods of producing single-wall nanotubes of greater purity and homogeneity. Furthermore, applications could make use of the properties of single-wall carbon nanotubes if only they were available in a form where they were attached directly to the surface of a macroscopic object. However, such components have not been produced up to now.
A method of producing carbon fibers from single-wall carbon nanotubes is described in PCT Patent Application No. PCT/US98/04513, incorporated herein by reference in its entirety. The single-wall nanotube molecules are produced in substantially two-dimensional array made up of single-walled nanotubes aggregating (e.g., by van der Waals forces) in substantially parallel orientation to form a monolayer extending in directions substantially perpendicular to the orientation of the individual nanotubes. Such monolayer arrays can be formed by conventional techniques employing “self-assembled monolayers” (SAM) or Langmiur-Blodgett films, see Hirch, pp. 75–76.
Typically, SAMs are created on a substrate which can be a metal (such as gold, mercury or ITO (indium-tin-oxide)). The molecules of interest, here the single-wall nanotube molecules, are linked (usually covalently) to the substrate through a linker moiety such as —S—, —S—(CH2)n—NH—, —SiO3(CH2)3NH— or the like. The linker moiety may be bound first to the substrate layer or first to the single-wall nanotube molecule (at an open or closed end) to provide for reactive self-assembly. Langmiur-Blodgett films are formed at the interface between two phases, e.g., a hydrocarbon (e.g., benzene or toluene) and water. Orientation in the film is achieved by employing molecules or linkers that have hydrophilic and lipophilic moieties at opposite ends.
The production of single-wall carbon nanotubes by metal-catalyzed disproportionation of carbon monoxide has been reported. See Dai, et al. (1996), Chem. Phys. Lett., 260:471–475. Preformed catalyst particles were made from a 50:50 mixture of Ni/Co supported on fumed alumina nanoparticles using known methods (See Int. Pat. WO 89/07163 (1989)). The diameter of the single- or multi-wall nanotube structure growing from a catalyst particle is related to the dimensions of the catalyst particle itself. Using the known methods of catalyst particle preparation, it is not possible to provide nanoparticles with a uniform optimum size to produce only single-wall nanotubes, and the growth process of Dai, et al., does not provide high yields of single-wall nanotubes because the larger particles produce multiwall nanotubes.